1. Field of the Invention
The present invention relates to a backlight unit emitting surface light, and a liquid crystal display using the same.
2. Description of the Related Art
In the recent information society, the emphasis has been put on the importance of display devices as visual information transfer media than ever. The cathode ray tubes, which are mostly used as the current display devices, have a problem in that they are heavy and bulky. The flat panel display devices such as plasma display devices, electro-luminescent display devices, liquid crystal displays (LCDs), etc. capable of the limits of the cathode ray tubes are being developed. Among these flat panel display devices, the LCDs show a tendency that their application fields are gradually increased due to the advantages of light weight, thin thickness, low-power-consumption driving, and so on.
These LCDs adjust an amount of transmitted light on the basis of image signals applied to a plurality of control switches arranged in a matrix pattern, thereby displaying an image corresponding to the image signal. At this time, a quantity of light transmitting the LCD is adjusted by orientation of liquid crystals in a liquid crystal panel, and thereby a gray scale is expressed.
The LCDs require a light source such as a backlight unit, because they are not spontaneous light-emitting display devices. The backlight unit for the LCDs is divided into a direct type and an edge type. The edge-type backlight unit comprises a light source and a light guide plate, which are disposed below the liquid crystal panel. The light guide plate guides light from the light source to the rear of the liquid crystal panel in a two-dimensional shape. Meanwhile, the direct-type backlight unit has a plurality of light sources disposed below the liquid crystal panel. The light sources emit light throughout the rear surface of the liquid crystal panel.
One of the light sources of the backlight unit includes a fluorescent lamp. The fluorescent lamp is classified as a cold cathode fluorescent lamp (CCFL) that is supplied with electric power through electrodes inserted into opposite ends of a glass tube, and an external electrode fluorescent lamp (EEFL) that is supplied with electric power through electrodes enclosing the outside of opposite ends of a glass tube using a metal material.
For example, the edge-type backlight unit using the fluorescent lamp is constructed as in FIG. 1. The edge-type backlight unit includes a fluorescent lamp 6 installed at one side (i.e. at an incident light side) of a light guide plate 2, and a lamp housing 4 enclosing the fluorescent lamp 6 such that only the incident light side of the light guide plate 2 is exposed. The light guide plate 2 causes the light from the fluorescent lamp 6 to travel through a surface thereof in a two-dimensional shape.
The lamp housing 4 reflects some of the light emitted from the fluorescent lamp 6 and traveling toward the side opposite to the incident light side of the light guide plate 2 toward the incident light side of the light guide plate 2, thereby increasing efficiency of the light of the backlight unit. In order to fully perform this function, the lamp housing 4 is produced so as to increase a quantity of light reflected toward the incident light side of the light guide plate 2 and simultaneously to decrease a quantity of light absorbed into the glass tube of the fluorescent lamp 6.
In this backlight unit, the fluorescent lamp 6 has no alternative but to be driven in a high temperature state due to heat generated by itself and a space closed by the lamp housing 4 and the light guide plate 2. Such a high temperature state (or condition) incurs reduction of light emitting efficiency and life span of the fluorescent lamp 6. Further, the high temperature state around the fluorescent lamp 6 locally changes the properties of optical sheets in the backlight unit including the light guide plate. For this reason, the surface light irradiated from the backlight unit is not uniform. In addition, the non-uniform surface light causes the brightness of an image displayed by the liquid crystal panel to become non-uniform. As a result, a quality of image displayed by the LCD is inevitably deteriorated.